Issue 19

P. K. Pradhan et alii, Frattura ed Integrità Strutturale, 19 (2012) 51-60; DOI: 10.3221/IGF-ESIS.19.05 56 It was not possible to precisely locate the exact position of void nucleation site using the optical microscope. Fig 9 shows the microphotograph of etched specimen strained at 12%. Voids are nucleated at the grain boundaries rich in pearlite as evident by the dark regions at the grain boundaries. In the present investigation, the elongation of the grains along with void formation at the grain boundaries is observed in Fig. 10 (a) and (b). It is clear that individual ferrite grains elongate parallel to the load axis during the tensile deformation. This is also evident from the stretch factor (SF) values of the image analysis results shown in Tab. 1. Ashby and Verrall [15] had proposed a mechanism of deformation in poly crystalline material in which the poly crystals were assumed to deform by disassembling in to constituent grains and then deforming. The deformation consists simultaneous action of grain elongation and rotation. During this process, overlapping of individual grains as well as void formation at the grain boundaries occurs which are subsequently taken care of statistically stored dislocations and geometrically necessary dislocations. (a) (b) Figure 10 : Elongated grain at (a) 20% strain and (b) 40% strain. (The tensile load axis is horizontal and vertical respectively for the above figures). Sl. No. Strain (%) Grain Size (A)ASTM No. Stretch factor Density after deformation (Kg/m³) % reduction in E’s value Actual Void volume fraction (%) Observed void volume fraction (%) 1 0 8.15 0.82 7883.4 --- 0.12 0.12 2 4 8.11 0.84 7863.57 9.237 0.371 0.25 3 8 8.16 0.85 7843.26 13.79 0.625 0.26 4 12 8.17 0.88 7820.4 19.10 0.910 0.54 5 16 8.13 0.91 7795.6 20.65 1.232 0.83 6 25 8.15 0.94 7762.25 22.02 1.6549 1.67 7 30 8.12 0.95 7762.0 24.81 1.66 1.214 8 40 (Fracture) 8.11 0.96 7723.5 ---- ---- ----- Table 1 : Structural changes of specimen for various strains (%). Tab. 2 shows that when strain (%) increases, the stretch factor increases. It indicates that the grains are elongated due to deformation of the specimen. But the effect of strain (%) on grain size (ASTM No.) is not significant in this test. The density of the steel, percentage decrease in the E’s value, the observed void volume fraction (from the Image analysis data) and actual void volume fraction calculated as per the rule of mixtures (from the density data) for the steel, obtained from individual specimen after straining to the specified values are shown. It is observed from Tab. 2 that with increase of